Distributed power system using direct current power sources

ABSTRACT

A distributed power system including multiple (DC) batteries each DC battery with positive and negative poles. Multiple power converters are coupled respectively to the DC batteries. Each power converter includes a first terminal, a second terminal, a third terminal and a fourth terminal. The first terminal is adapted for coupling to the positive pole. The second terminal is adapted for coupling to the negative pole. The power converter includes: (i) a control loop adapted for setting the voltage between or current through the first and second terminals, and (ii) a power conversion portion adapted to selectively either: convert power from said first and second terminals to said third and fourth terminals to discharge the battery connected thereto, or to convert power from the third and fourth terminals to the first and second terminals to charge the battery connected thereto. Each of the power converters is adapted for serial connection to at least one other power converter by connecting respectively the third and fourth terminals, thereby forming a serial string. A power controller is adapted for coupling to the serial string. The power controller includes a control part adapted to maintain current through or voltage across the serial string at a predetermined value.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/078,011, filed Nov. 12, 2013, which is a continuation ofU.S. patent application Ser. No. 12/911,153, filed Oct. 25, 2010 (nowissued as U.S. Pat. No. 8,618,692), and entitled “Distributed PowerSystem Using Direct Current Power Sources,” which is acontinuation-in-part of U.S. patent application Ser. No. 11/950,271,filed Dec. 4, 2007 (now issues as U.S. Pat. No. 9,088,178), and entitled“Distributed Power Harvesting Systems Using DC Power Sources,” and whichclaims the benefit of U.S. Provisional Patent Application No.61/254,681, filed Oct. 24, 2009, and entitled “Distributed ConverterArchitecture For Battery Banks,” each of which are incorporated byreference herein in their entirety for all purposes.

BACKGROUND

1. Technical Field

The field of the invention relates generally to power production fromdistributed DC power sources, and more particularly to management ofdistributed DC power sources in series installations.

2. Description of Related Art

The recent increased interest in renewable energy has led to increasedresearch in systems for distributed generation of energy, such asphotovoltaic cells (PV), fuel cells, batteries (e.g., for hybrid cars),etc. Various topologies have been proposed for connecting these powersources to the load, taking into consideration various parameters, suchas voltage/current requirements, operating conditions, reliability,safety, costs, etc. For example, most of these sources provide lowvoltage output (normally a few volts for one cell, or a few tens ofvolts for serially connected cells), so that many of them need to beconnected serially to achieve the required operating voltage.Conversely, a serial connection may fail to provide the requiredcurrent, so that several strings of serial connections may need to beconnected in parallel to provide the required current.

It is also known that power generation from each of these sourcesdepends on manufacturing, operating, and environmental conditions. Forexample, various inconsistencies in manufacturing may cause twoidentical sources to provide different output characteristics.Similarly, two identical sources may react differently to operatingand/or environmental conditions, such as load, temperature, etc. Inpractical installations, different source may also experience differentenvironmental conditions, e.g., in solar power installations some panelsmay be exposed to full sun, while others be shaded, thereby deliveringdifferent power output. In a multiple-battery installation, some of thebatteries may age differently, thereby delivering different poweroutput. While these problems and the solutions provided by the subjectinvention are applicable to any distributed power system, the followingdiscussion turns to solar energy so as to provide better understandingby way of a concrete example.

A conventional installation of solar power system 10 is illustrated inFIG. 1. Since the voltage provided by each individual solar panel 101 islow, several panels are connected in series to form a string of panels103. For a large installation, when higher current is required, severalstrings 103 may be connected in parallel to form the overall system 10.The solar panels are mounted outdoors, and their leads are connected toa maximum power point tracking (MPPT) module 107 and then to an inverter104. The MPPT 107 is typically implemented as part of the inverter 104.The harvested power from the DC sources is delivered to the inverter104, which converts the fluctuating direct-current (DC) intoalternating-current (AC) having a desired voltage and frequency, whichis usually 110V or 220V at 60 Hz, or 220V at 50 Hz (It is interesting tonote the even in the US many inverters produce 220V, which is then splitinto two 110V feeds in the electric box). The AC current from theinverter 104 may then be used for operating electric appliances or fedto the power grid. Alternatively, if the installation is not tied to thegrid, the power extracted from the inverter may be directed to aconversion and charge/discharge circuit to store the excess powercreated as charge in batteries. In case of a battery-tied application,the inversion stage might be skipped altogether, and the DC output ofthe MPPT stage 107 may be fed into the charge/discharge circuit.

As noted above, each solar panel 101 supplies relatively very lowvoltage and current. The problem facing the solar array designer is toproduce a standard AC current at 120V or 220V root-mean-square (RMS)from a combination of the low voltages of the solar panels. The deliveryof high power from a low voltage requires very high currents, whichcause large conduction losses on the order of the second power of thecurrent (I²). Furthermore, a power inverter, such as the inverter 104,which is used to convert DC current to AC current, is most efficientwhen its input voltage is slightly higher than its output RMS voltagemultiplied by the square root of 2. Hence, in many applications, thepower sources, such as the solar panels 101, are combined in order toreach the correct voltage or current. The most common method connectsthe power sources in series in order to reach the desirable voltage andin parallel in order to reach the desirable current, as shown in FIG. 1.A large number of the panels 101 are connected into a string 103 and thestrings 103 are connected in parallel to the power inverter 104. Thepanels 101 are connected in series in order to reach the minimal voltagerequired for the inverter. Multiple strings 103 arc connected inparallel into an array to supply higher current, so as to enable higherpower output.

While this configuration is advantageous in terms of cost andarchitecture simplicity, several drawbacks have been identified in theliterature for such architecture. One recognized drawback isinefficiencies cause by non-optimal power draw from each individualpanel, as explained below. As explained above, the output of the DCpower sources is influenced by many conditions. Therefore, to maximizethe power draw from each source, one needs to draw the combination ofvoltage and current that provides the peak power for the currentlyprevailing conditions. As conditions change, the combination of voltageand current draw may need to be changed as well.

FIG. 2 illustrates one serial string of DC sources, e.g., solar panels201 a-201 d, connected to MPPT circuit 207 and inverter 204. The currentversus voltage (IV) characteristics plotted (210 a-210 d) to the left ofeach DC source 201. For each DC source 201, the current decreases as theoutput voltage increases. At some voltage value the current goes tozero, and in some applications may assume a negative value, meaning thatthe source becomes a sink. Bypass diodes are used to prevent the sourcefrom becoming a sink. The power output of each source 201, which isequal to the product of current and voltage (P=I*V), varies depending onthe voltage drawn from the source. At a certain current and voltage,close to the falling off point of the current, the power reaches itsmaximum. It is desirable to operate a power generating cell at thismaximum power point. The purpose of the MPPT is to find this point andoperate the system at this point so as to draw the maximum power fromthe sources.

In a typical, conventional solar panel array, different algorithms andtechniques are used to optimize the integrated power output of thesystem 10 using the MPPT module 107. The MPPT module 107 receives thecurrent extracted from all of the solar panels together and tracks themaximum power point for this current to provide the maximum averagepower such that if more current is extracted, the average voltage fromthe panels starts to drop, thus lowering the harvested power. The MPPTmodule 107 maintains a current that yields the maximum average powerfrom the overall system 10.

However, since the sources 201 a-201 d are connected in series to asingle MPPT 207, the MPPT must select a single point, which would besomewhat of an average of the MPP of the serially connected sources. Inpractice, it is very likely that the MPPT would operate at an I-V pointthat is optimum to only a few or none of the sources. In the example ofFIG. 2, the selected point is the maximum power point for source 201 b,but is off the maximum power point for sources 201 a, 201 c and 201 d.Consequently, the arrangement is not operated at best achievableefficiency.

Turning back to the example of a solar system 10 of FIG. 1, fixing apredetermined constant output voltage from the strings 103 may cause thesolar panels to supply lower output power than otherwise possible.Further, each string carries a single current that is passed through allof the solar panels along the string. If the solar panels are mismatcheddue to manufacturing differences, aging or if they malfunction or areplaced under different shading conditions, the current, voltage andpower output of each panel will be different. Forcing a single currentthrough all of the panels of the string causes the individual panels towork at a non-optimal power point and can also cause panels which arehighly mismatched to generate “hot spots” due to the high currentflowing through them. Due to these and other drawbacks of conventionalcentralized methods, the solar panels have to be matched properly. Insome cases external diodes are used to bypass the panels that are highlymismatched. In conventional multiple string configurations all stringshave to be composed of exactly the same number of solar panels and thepanels are selected of the same model and must be install at exactly thesame spatial orientation, being exposed to the same sunlight conditionsat all times. This is difficult to achieve and can be very costly.

BRIEF SUMMARY

According to embodiments of the present invention there is provided adistributed power system including multiple (DC) batteries each DCbattery with positive and negative poles. Multiple power converters arecoupled respectively to the DC batteries. Each power converter includesa first terminal, a second terminal, a third terminal and a fourthterminal. The first terminal is adapted for coupling to the positivepole. The second terminal is adapted for coupling to the negative pole.The power converter includes: (i) a control loop adapted for setting thevoltage between or current through the first and second terminals, and(ii) a power conversion portion adapted to selectively either: convertpower from said first and second terminals to said third and fourthterminals to discharge the battery connected thereto, or to convertpower from the third and fourth terminals to the first and secondterminals to charge the battery connected thereto.

Each of the power converters is adapted for serial connection to atleast one other power converter by connecting respectively the third andfourth terminals, thereby forming a serial string. A power controller isadapted for coupling to the serial string. The power controller includesa control part adapted to maintain current through or voltage across theserial string at a predetermined value. The control part may maintainvoltage across the serial string at a predetermined value or the controlpart may maintain current through the serial string at a predeterminedvalue. The power controller may include a bidirectional DC/AC inverteror bi-directional DC/DC converter. The power converters may function asa current source, voltage regulator or trickle charge source. Thedistributed power system may further include multiple photovoltaicpanels; multiple DC-DC converters. Each of the DC-to-DC converters mayinclude input terminals coupled to a respective DC photovoltaic panelsand output terminals coupled in series to the other DC-to-DC converters,thereby forming a second serial string. A control loop sets the voltageand/or current at the input terminals of the DC-to-DC converteraccording to predetermined criteria. A power conversion portion convertsthe power received at the input terminals to an output power at theoutput terminals. The serial string and the second serial string areconnectable in parallel to form parallel-connected strings. A powercontroller may be adapted for coupling in parallel to theparallel-connected strings, the power controller including a controlpart adapted to maintain current through or voltage across the parallelconnected strings at a predetermined value. The power controller may beoff-grid (not connected to the grid) or connected to the grid. Thephotovoltaic panels may provide electrical power for charging thebatteries.

According to embodiments of the present invention there is provided adistributed power system including multiple (DC) batteries each DCbattery with positive and negative poles. Multiple power converters arecoupled respectively to the DC batteries. Each power converter includesa first terminal, a second terminal, a third terminal and a fourthterminal. The first terminal is adapted for coupling to the positivepole. The second terminal is adapted for coupling to the negative pole.The power converter includes a first control loop configured to seteither current through or voltage between the first and secondterminals, and a second control loop configured set either currentthrough or voltage between the third and fourth terminals; and (iii) apower conversion portion adapted to selectively either: convert powerfrom the first and second terminals to the third and fourth terminals todischarge the battery connected thereto, or to convert power from thethird and fourth terminals to the first and second terminals to chargethe battery connected thereto; wherein each of the power converters isadapted for serial connection to at least one other power converter byconnecting respectively the third and fourth terminals, thereby forminga serial string. The distributed power system may further includemultiple photovoltaic panels and multiple DC-DC converters. Each of theDC-to-DC converters may include input terminals coupled to a respectiveDC photovoltaic panels and output terminals coupled in series to theother DC-to-DC converters, thereby forming a second serial string. Acontrol loop sets the voltage and/or current at the input terminals ofthe DC-to-DC converter according to predetermined criteria. A powerconversion portion converts the power received at the input terminals toan output power at the output terminals. The serial string and thesecond serial string are connectable in parallel to formparallel-connected strings. The power controller is selectably eitheroff-grid or connected to grid. The photovoltaic panels may provideelectrical power for charging the batteries. A communications interfacebetween the power controller and the power converters may be used forcontrolling charging and discharging of the batteries.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 illustrates a conventional centralized power harvesting systemusing DC power sources.

FIG. 2 illustrates current versus voltage characteristic curves for oneserial string of DC sources.

FIG. 3 illustrates a distributed power harvesting system, according toaspects of the invention, using DC power sources.

FIGS. 3a-3c show variations of distributed power systems using DCbatteries according to a different embodiments of the present invention.

FIGS. 4A and 4B illustrate the operation of the system of FIG. 3 underdifferent conditions, according to aspects of the invention.

FIG. 4C illustrates an embodiment of the invention wherein the invertercontrols the input current.

FIG. 5 illustrates a distributed power harvesting system, according toother aspects of the invention, using DC power sources.

FIG. 6 illustrates an exemplary DC-to-DC converter according to aspectsof the invention.

FIG. 6a shows a slightly modified DC-DC converter based on the DC-DCconverter shown in FIG. 6, according to an embodiment of the presentinvention.

FIG. 7 illustrates a power converter, according to aspects of theinvention including control features of the aspects of the invention.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to the like elementsthroughout. The embodiments are described below to explain the presentinvention by referring to the figures.

Before explaining embodiments of the invention in detail, it is to beunderstood that the invention is not limited in its application to thedetails of design and the arrangement of the components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of other embodiments or of being practiced or carried out invarious ways. Also, it is to be understood that the phraseology andterminology employed herein is for the purpose of description and shouldnot be regarded as limiting.

The topology provided by the subject invention solves many of theproblems associated with, and has many advantages over, the conventionalart topologies. For example, the inventive topology enables seriallyconnecting mismatched power sources, such as mismatched solar panels,panel of different models and power ratings, and even panels fromdifferent manufacturers and semiconductor materials. It allows serialconnection of sources operating under different conditions, such as,e.g., solar panels exposed to different light or temperature conditions.It also enables installations of serially connected panels at differentorientations or different sections of the roof or structure. This andother features and advantages will become apparent from the followingdetailed description. Aspects of the present invention provide a systemand method for combining power from multiple DC power sources into asingle power supply. According to aspects of the present invention, eachDC power source is associated with a DC-DC power converter. Modulesformed by coupling the DC power sources to their associated convertersare coupled in series to provide a string of modules. The string ofmodules is then coupled to an inverter having its input voltage fixed. Amaximum power point control loop in each converter harvests the maximumpower from each DC power source and transfers this power as output fromthe power converter. For each converter, substantially all the inputpower is converted to the output power, such that the conversionefficiency may be 90% or higher in some situations. Further, thecontrolling is performed by fixing the input current or input voltage ofthe converter to the maximum power point and allowing output voltage ofthe converter to vary. For each power source, one or more sensorsperform the monitoring of the input power level to the associatedconverter. In some aspects of the invention, a microcontroller mayperform the maximum power point tracking and control in each converterby using pulse width modulation to adjust the duty cycle used fortransferring power from the input to the output.

One aspect of the present invention provides a greater degree of faulttolerance, maintenance and serviceability by monitoring, logging and/orcommunicating the performance of each solar panel. In one aspect of theinvention, the microcontroller that is used for maximum power pointtracking, may also be used to perform the monitoring, logging andcommunication functions. These functions allow for quick and easytroubleshooting during installation, thereby significantly reducinginstallation time. These functions are also beneficial for quickdetection of problems during maintenance work. Aspects of the presentinvention allow easy location, repair, or replacement of failed solarpanels. When repair or replacement is not feasible, bypass features ofthe current invention provide increased reliability.

In one aspect, the present invention relates to arrays of solar cellswhere the power from the cells is combined. Each converter may beattached to a single solar cell, or a plurality of cell connected inseries, in parallel, or both, e.g., parallel connection of strings ofserially connected cells. In one embodiment each converter is attachedto one panel of photovoltaic strings. However, while applicable in thecontext of solar power technology, the aspects of the present inventionmay be used in any distributed power network using DC power sources. Forexample, they may be used in batteries with numerous cells or hybridvehicles with multiple fuel cells on board The DC power sources may besolar cells, solar panels, electrical fuel cells, electrical batteries,and the like. Further, although the discussion below relates tocombining power from an array of DC power sources into a source of ACvoltage, the aspects of the present invention may also apply tocombining power from DC sources into another DC voltage.

FIG. 3 illustrates a distributed power harvesting configuration 30,according to an embodiment of the present invention. Configuration 30enables connection of multiple power sources, for example solar panels301 a-301 d, to a single power supply. In one aspect of the invention,the series string of all of the solar panels may be coupled to aninverter 304. In another aspect of the invention, several seriallyconnected strings of solar panels may be connected to a single inverter304. The inverter 304 may be replaced by other elements, such as, e.g.,a charging regulator for charging a battery bank.

In configuration 30, each solar panel 301 a-301 d is connected to aseparate power converter circuit 305 a-305 d. One solar panel togetherwith its associated power converter circuit forms a module, e.g., module320. Each converter 305 a-305 d adapts optimally to the powercharacteristics of the connected solar panel 301 a-301 d and transfersthe power efficiently from converter input to converter output. Theconverters 305 a-305 d can be buck converters, boost converters,buck/boost converters, flyback or forward converters, etc. Theconverters 305 a-305 d may also contain a number of componentconverters, for example a serial connection of a buck and a boostconverter. Each converter 305 a-305 d includes a control loop 323 i thatreceives a feedback signal, not from the converter's output current orvoltage, but rather from the converter's input coming from the solarpanel 301. An example of such a control loop is a maximum power pointtracking (MPPT) loop. The MPPT loop in the converter locks the inputvoltage and current from each solar panel 301 a-301 d to its optimalpower point.

Conventional DC-to-DC converters may have a wide input voltage range attheir input and an output voltage that is predetermined and fixed. Inthese conventional DC-to-DC voltage converters, a controller within theconverter monitors the current or voltage at the input, and the voltageat the output. The controller determines the appropriate pulse widthmodulation (PWM) duty cycle to fix the output voltage to thepredetermined value by increasing the duty cycle if the output voltagedrops. Accordingly, the conventional converter includes a feedback loopthat closes on the output voltage and uses the output voltage to furtheradjust and fine tune the output voltage from the converter. As a resultof changing the output voltage, the current extracted from the input isalso varied.

In the converters 305 a-305 d, according to aspects of the presentinvention, a controller within the converter 405 monitors the voltageand current at the converter input and determines the PWM in such a waythat maximum power is extracted from the attached panel 301 a-301 d. Thecontroller of the converter 405 dynamically tracks the maximum powerpoint at the converter input. In the aspects of the present invention,the feedback loop is closed on the input power in order to track maximuminput power rather than closing the feedback loop on the output voltageas performed by conventional DC-to-DC voltage converters.

As a result of having a separate MPPT circuit in each converter 305a-305 d, and consequently for each solar panel 301 a-301 d, each string303 in the embodiment shown in FIG. 3 may have a different number ordifferent brand of panels 301 a-301 d connected in series. The circuitof FIG. 3 continuously performs MPPT on the output of each solar panel301 a-301 d to react to changes in temperature, solar radiance, shadingor other performance factors that impact that particular solar panel 301a-301 d. As a result, the MPPT circuit within the converters 305 a-305 dharvests the maximum possible power from each panel 301 a-301 d andtransfers this power as output regardless of the parameters impactingthe other solar panels.

As such, the aspects of the invention shown in FIG. 3 continuously trackand maintain the input current and the input voltage to each converterat the maximum power point of the DC power source providing the inputcurrent and the input voltage to the converter. The maximum power of theDC power source that is input to the converter is also output from theconverter. The converter output power may be at a current and voltagedifferent from the converter input current and voltage. The outputcurrent and voltage from the converter are responsive to requirements ofthe series connected portion of the circuit.

In one aspect of the invention, the outputs of converters 305 a-305 dare series connected into a single DC output that forms the input to theload or power supplier, in this example, inverter 304. The inverter 304converts the series connected DC output of the converters into an ACpower supply. The load, in this case inverter 304, regulates the voltageat the load's input. That is, in this example, an independent controlloop 321 holds the input voltage at a set value, say 400 volts.Consequently, the inverter's input current is dictated by the availablepower, and this is the current that flows through all serially connectedDC sources. On the other hand, while the output of the DC-DC convertersmust be at the inverter's current input, the current and voltage inputto the converter is independently controlled using the MPPT.

In the conventional art, the input voltage to the load was allowed tovary according to the available power. For example, when a lot ofsunshine is available in a solar installation, the voltage input to theinverter can vary even up to 1000 volts. Consequently, as sunshineillumination varies, the voltage varies with it, and the electricalcomponents in the inverter (or other power supplier or load) are exposedto varying voltage. This tends to degrade the performance of thecomponents and ultimately causes them to fail. On the other hand, byfixing the voltage or current to the input of the load or powersupplier, here the inverter, the electrical components are alwaysexposed to the same voltage or 30 current and therefore would haveextended service life. For example, the components of the load (e.g.,capacitors, switches and coil of the inverter) may be selected so thatat the fixed input voltage or current they operate at, say, 60% of theirrating. This would improve the reliability and prolong the service lifeof the component, which is critical for avoiding loss of service inapplications such as solar power systems.

FIGS. 4A and 4B illustrate the operation of the system of FIG. 3 underdifferent conditions, according to aspects of the invention. Theexemplary configuration 40 is similar to configuration 30 of FIG. 3. Inthe example shown, ten DC power sources 401/1 through 401/10 areconnected to ten power converters 405/1 through 405/10, respectively.The modules formed by the DC power sources and their correspondingconverters arc coupled together in series to form a string 403. In oneaspect of the invention, the series-connected converters 405 are coupledto a DC-to-AC inverter 404.

The DC power sources may be solar panels and the example is discussedwith respect to solar panels as one illustrative case. Each solar panel401 may have a different power output due to manufacturing tolerances,shading, or other factors. For the purpose of the present example, anideal case is illustrated in FIG. 4A, where efficiency of the DC-to-DCconversion is assumed to be 100% and the panels 501 are assumed to beidentical. In some aspects of the invention, efficiencies of theconverters may be quite high and range at about 95%-99%. So, theassumption of 100% efficiency is not unreasonable for illustrationpurposes. Moreover, according to embodiments of the subject invention,each of the DC-DC converters are constructed as a power converter, i.e.,it transfers to its output the entire power it receives in its inputwith very low losses.

Power output of each solar panel 401 is maintained at the maximum powerpoint for the panel by a control loop within the corresponding powerconverter 405. In the example shown in FIG. 4A, all of the panels areexposed to full sun illumination and each solar panel 401 provides 200 Wof power. Consequently, the MPPT loop will draw current and voltagelevel that will transfer the entire 200 W from the panel to itsassociated converter.

That is, the current and voltage dictated by the MPPT form the inputcurrent I_(in), and input voltage V_(in) to the converter. The outputvoltage is dictated by the constant voltage set at the inverter 404, aswill be explained below. The output current I_(out) would then be thetotal power, i.e., 200 W, divided by the output voltage V_(out).

As noted above, according to a feature of the invention, the inputvoltage to inverter 404 is controlled by the inverter (in this example,kept constant), by way of control loop 421. For the purpose of thisexample, assume the input voltage is kept as 400V (ideal value forinverting to 220 VAC). Since we assume that there are ten seriallyconnected power converters, each providing 200 W, we can see that theinput current to the inverter 404 is 2000 W/400V=5 A. Thus, the currentflowing through each of the converters 401/1-401/10 must be 5 A. Thismeans that in this idealized example each of the converters provides anoutput voltage of 200 W/5 A=40V. Now, assume that the MPPT for eachpanel (assuming perfect matching panels) dictates V MPP=32V. This meansthat the input voltage to the inverter would be 32V, and the inputcurrent would be 200 W/32V=6.25 A.

We now turn to another example, wherein the system is still maintainedat an ideal mode (i.e., perfectly matching DC sources and entire poweris transferred to the inverter), but the environmental conditions arenot ideal. For example, one DC source is overheating, is malfunctioning,or, as in the example of FIG. 4B, the ninth solar panel 401/9 is shadedand consequently produces only 40 W of power. Since we keep all otherconditions as in the example of FIG. 4A, the other nine solar panels 401are unshaded and still produce 200 W of power. The power converter 405/9includes MPPT to maintain the solar panel 501/9 operating at the maximumpower point, which is now lowered due to the shading.

The total power available from the string is now 9×200 W+40 W=1840 W.Since the input to the inverter is still maintained at 400V, the inputcurrent to the inverter will now be 1840 W/40V=4.6 A. This means thatthe output of all of the power converters 405/1-405/10 in the stringmust be at 4.6 A. Therefore, for the nine unshaded panels, theconverters will output 200 W/4.6 A=43.5V. On the other hand, theconverter 405/9 attached to the shaded panel 401/9 will output 40 W/4.6A=8.7V. Checking the math, the input to the inverter can be obtained byadding nine converters providing 43.5V and one converter providing 8.7V,i.e., (9×43.5V)+8.7V=400V.

The output of the nine non-shaded panels would still be controlled bythe MPPT as in FIG. 4A, thereby standing at 32V and 6.25 A. On the otherhand, since the nines panes 401/9 is shaded, let's assume its MPPTdropped to 28V. Consequently, the output current of the ninth panel is40 W/28V=1.43 A. As can be seen by this example, all of the panels areoperated at their maximum power point, regardless of operatingconditions. As shown by the example of FIG. 4B, even if the output ofone DC source drops dramatically, the system still maintains relativelyhigh power output by fixing the voltage input to the inverter, andcontrolling the input to the converters independently so as to drawpower from the DC source at the MPP.

As can be appreciated, the benefit of the topology illustrated in FIGS.4A and 4B are numerous. For example, the output characteristics of theserially connected DC sources, such as solar panels, need not match.Consequently, the serial string may utilize panels from differentmanufacturers or panels installed on different parts of the roofs (i.e.,at different spatial orientation). Moreover, if several strings areconnected in parallel, it is not necessary that the strings match;rather each string may have different panels or different number ofpanels. This topology also enhances reliability by alleviating the hotspot problem. That is, as shown in FIG. 4A the output of the shadedpanel 401/9 is 1.43 A, while the current at the output of the unshadedpanels is 6.25 A. This discrepancy in current when the components areseries connected causes a large current being forced through the shadedpanel that may cause overheating and malfunction at this component.However, by the inventive topology wherein the input voltage is setindependently, and the power draw from each panel to its converter isset independently according to the panels MPP at each point in time, thecurrent at each panel is independent on the current draw from theserially connected converters.

It is easily realized that since the power is optimized independentlyfor each panel, panels could be installed in different facets anddirections in BIPV installations. Thus, the problem of low powerutilization in building-integrated installations is solved, and moreinstallations may now be profitable.

The described system could also easily solve the problem of energyharvesting in low light conditions. Even small amounts of light areenough to make the converters 405 operational, and they then starttransferring power to the inverter. If small amounts of power areavailable, there will be a low current flow—but the voltage will be highenough for the inverter to function, and the power will indeed beharvested.

According to aspects of the invention, the inverter 404 includes acontrol loop 421 to maintain an optimal voltage at the input of inverter404. In the example of FIG. 4B, the input voltage to inverter 404 ismaintained at 400V by the control loop 421. The converters 405 aretransferring substantially all of the available power from the solarpanels to the input of the inverter 404. As a result, the input currentto the inverter 404 is dependent only on the power provided by the solarpanels and the regulated set, i.e., constant, voltage at the inverterinput.

The conventional inverter 104, shown in FIG. 1 and FIG. 3, is requiredto have a very wide input voltage to accommodate for changingconditions, for example a change in luminance, temperature and aging ofthe solar array. This is in contrast to the inverter 404 that isdesigned according to aspects of the present invention. The inverter 404does not require a wide input voltage and is therefore simpler to designand more reliable. This higher reliability is achieved, among otherfactors, by the fact that there are no voltage spikes at the input tothe inverter and thus the components of the inverter experience lowerelectrical stress and may last longer.

When the inverter 404 is a part of the circuit, the power from thepanels is transferred to a load that may be connected to the inverter.To enable the inverter 404 to work at its optimal input voltage, anyexcess power produced by the solar array, and not used by the load, isdissipated. Excess power may be handled by selling the excess power tothe utility company if such an option is available. For off-grid solararrays, the excess power may be stored in batteries. Yet another optionis to connect a number of adjacent houses together to form a micro-gridand to allow load-balancing of power between the houses. If the excesspower available from the solar array is not stored or sold, then anothermechanism may be provided to dissipate excess power.

The features and benefits explained with respect to FIGS. 4A and 4Bstem, at least partially, from having the inverter dictates the voltageprovided at its input. Conversely, a design can be implemented whereinthe inverter dictates the current at its input. Such an arrangement isillustrated in FIG. 4C. FIG. 4C illustrates an embodiment of theinvention wherein the inverter controls the input current. Power outputof each solar panel 401 is maintained at the maximum power point for thepanel by a control loop within the corresponding power converter 405. Inthe example shown in FIG. 4C, all of the panels are exposed to full sunillumination and each solar panel 401 provides 200 W of power.Consequently, the MPPT loop will draw current and voltage level thatwill transfer the entire 200 W from the panel to its associatedconverter. That is, the current and voltage dictated by the MPPT formthe input current I_(in) and input voltage V_(in) to the converter. Theoutput voltage is dictated by the constant current set at the inverter404, as will be explained below. The output voltage V_(out) would thenbe the total power, i.e., 200 W, divided by the output current I_(out).

As noted above, according to a feature of the invention, the inputcurrent to inverter 404 is dictated by the inverter by way of controlloop 421. For the purpose of this example, assume the input current iskept as 5 A. Since we assume that there are ten serially connected powerconverters, each providing 200 W, we can see that the input voltage tothe inverter 404 is 2000 W/5 A=400V. Thus, the current flowing througheach of the converters 401/1-401/10 must be 5 A. This means that in thisidealized example each of the converters provides an output voltage of200 W/5 A=40V. Now, assume that the MPPT for each panel (assumingperfect matching panels) dictates V MPP=32V. This means that the inputvoltage to the inverter would be 32V, and the input current would be 10200 W/32V=6.25 A.

Consequently, similar advantages have been achieved by having theinverter control the current, rather than the voltage. However, unlikethe conventional art, changes in the output of the panels will not causein changes in the current flowing to the inverter, as that is dictatedby the inverter itself. Therefore, if the inverter is designed to keepthe current or the voltage constant, then regardless of the operation ofthe panels, the current or voltage to the inverter will remain constant.

FIG. 5 illustrates a distributed power harvesting system, according toother aspects of the invention, using DC power sources. FIG. 5illustrates multiple strings 503 coupled together in parallel. Each ofthe strings is a series connection of multiple modules and each of themodules includes a DC power source 501 that is coupled to a converter505. The DC power source may be a solar panel. The output of theparallel connection of the strings 503 is connected, again in parallel,to a shunt regulator 506 and a load controller 504. The load controller504 may be an inverter as with the embodiments of FIGS. 4A and 4B. Shuntregulators automatically maintain a constant voltage across itsterminals.

The shunt regulator 506 is configured to dissipate excess power tomaintain the input voltage at the input to the inverter 504 at aregulated level and prevent the inverter input voltage from increasing.The current which flows through shunt regulator 506 complements thecurrent drawn by inverter 504 in order to ensure that the input voltageof the inverter is maintained at a constant level, for example at 400V.

By fixing the inverter input voltage, the inverter input current isvaried according to the available power draw. This current is dividedbetween the strings 503 of the series connected converters. When eachconverter includes a controller loop maintaining the converter inputvoltage at the maximum power point of the associated DC power source,the output power of the converter is determined. The converter power andthe converter output current together determine the converter outputvoltage. The converter output voltage is used by a power conversioncircuit in the converter for stepping up or stepping down the converterinput voltage to obtain the converter output voltage from the inputvoltage as determined by the MPPT.

FIG. 6 illustrates an exemplary DC-to-DC converter 605 according toaspects of the invention. DC-to-DC converters arc conventionally used toeither step down or step up a varied or constant DC voltage input to ahigher or a lower constant voltage output, depending on the requirementsof the circuit. However, in the embodiment of FIG. 6 the DC-DC converteris used as a power converter, i.e., transferring the input power tooutput power, the input voltage varying according to the MPPT, while theoutput current being dictated by the constant input voltage to theinverter. That is, the input voltage and current may vary at any timeand the output voltage and current may vary at any time, depending onthe operating condition of the DC power sources.

The converter 605 is connected to a corresponding DC power source 601 atinput terminals 614 and 616. The converted power of the DC power source601 is output to the circuit through output terminals 610, 612. Betweenthe input terminals 614, 616 and the output terminals 610, 612, theremainder of the converter circuit is located that includes input andoutput capacitors 620, 640, back flow prevention diodes 622, 642 and apower conversion circuit including a controller 606 and an inductor 608.

The inputs 616 and 614 are separated by a capacitor 620 which acts as anopen to a DC voltage. The outputs 610 and 612 are also separated by acapacitor 640 that also acts an open to DC output voltage. Thesecapacitors are DC-blocking or AC-coupling capacitors that short whenfaced with alternating current of a frequency for which they areselected. Capacitor 640 coupled between the outputs 610, 612 and alsooperates as a part of the power conversion circuit discussed below.

Diode 642 is coupled between the outputs 610 and 612 with a polaritysuch that current may not backflow into the converter 605 from thepositive lead of the output 612. Diode 622 is coupled between thepositive output lead 612 through inductor 608 which acts a short for DCcurrent and the negative input lead 614 with such polarity to prevent acurrent from the output 612 to backflow into the solar panel 601.

The DC power sources 601 may be solar panels. A potential differenceexists between the wires 614 and 616 due to the electron-hole pairsproduced in the solar cells of panel 601. The converter 605 maintainsmaximum power output by extracting current from the solar panel 601 atits peak power point by continuously monitoring the current and voltageprovided by the panel and using a maximum power point trackingalgorithm. The controller 606 includes an MPPT circuit or algorithm forperforming the peak power tracking. Peak power tracking and pulse widthmodulation, PWM, are performed together to achieve the desired inputvoltage and current. The MPPT in the controller 606 may be anyconventional MPPT, such as, e.g, perturb and observe (P&O), incrementalconductance, etc. However, notably the MPPT is performed on the paneldirectly, i.e., at the input to the converter, rather than at the outputof the converter. The generated power is then transferred to the outputterminals 610 and 612. The outputs of multiple converters 605 may beconnected in series, such that the positive lead 612 of one converter605 is connected to the negative lead 610 of the next converter 605.

In FIG. 6, the converter 605 is shown as a buck plus boost converter.The term “buck plus boost” as used herein is a buck converter directlyfollowed by a boost converter as shown in FIG. 6, which may also appearin the literature as “cascaded buck-boost converter”. If the voltage isto be lowered, the boost portion is substantially shorted. If thevoltage is to be raised, the buck portion is substantially shorted. Theterm “buck plus boost” differs from buck/boost topology which is aclassic topology that may be used when voltage is to be raised orlowered. The efficiency of “buck/boost” topology is inherently lowerthan a buck or a boost. Additionally, for given requirements, abuck-boost converter will need bigger passive components then a buckplus boost converter in order to function. Therefore, the buck plusboost topology of FIG. 6 has a higher efficiency than the buck/boosttopology. However, the circuit of FIG. 6 continuously decides whether itis bucking or boosting. In some situations when the desired outputvoltage is similar to the input voltage, then both the buck and boostportions may be operational.

The controller 606 may include a pulse width modulator, PWM, or adigital pulse width modulator, DPWM, to be used with the buck and boostconverter circuits. The controller 606 controls both the buck converterand the boost converter and determines whether a buck or a boostoperation is to be performed. In some circumstances both the buck andboost portions may operate together. That is, as explained with respectto the embodiments of FIGS. 4A and 4B, the input voltage and current areselected independently of the selection of output current and voltage.Moreover, the selection of either input or output values may change atany given moment depending on the operation of the DC power sources.Therefore, in the embodiment of FIG. 6 the converter is constructed sothat at any given time a selected value of input voltage and current maybe up converted or down converted depending on the output requirement.

In one implementation, an integrated circuit (IC) 604 may be used thatincorporates some of the functionality of converter 605. IC 604 isoptionally a single ASIC able to withstand harsh temperature extremespresent in outdoor solar installations. ASIC 604 may be designed for ahigh mean time between failures (MTBF) of more than 25 years. However, adiscrete solution using multiple integrated circuits may also be used ina similar manner. In the exemplary embodiment shown in FIG. 6, the buckplus boost portion of the converter 605 is implemented as the IC 604.Practical considerations may lead to other segmentations of the system.For example, in one aspect of the invention, the IC 604 may include twoICs, one analog IC which handles the high currents and voltages in thesystem, and one simple low-voltage digital IC which includes the controllogic. The analog IC may be implemented using power FETs which mayalternatively be implemented in discrete components, FET drivers, A/Ds,and the like. The digital IC may form the controller 606.

In the exemplary circuit shown, the buck converter includes the inputcapacitor 620, transistors 628 and 630 a diode 622 positioned inparallel to transistor 628, and an inductor 608. The transistors 628,630 each have a parasitic body diode 624, 626. In the exemplary circuitshown, the boost converter includes the inductor 608, which is sharedwith the buck converter, transistors 648 and 650 a diode 642 positionedin parallel to transistor 650, and the output capacitor 640. Thetransistors 648, 650 each have a parasitic body diode 644, 646.

As shown in FIG. 1, adding electronic elements in the series arrangementmay reduce the reliability of the system, because if one electricalcomponent breaks it may affect the entire system. Specifically, if afailure in one of the serially connected elements causes an open circuitin the failed element, current ceases to flow through the entire series,thereby causing the entire system to stop function. Aspects of thepresent invention provide a converter circuit where electrical elementsof the circuit have one or more bypass routes associated with them thatcarry the current in case of the electrical element fails. For example,each switching transistor of either the buck or the boost portion of theconverter has its own bypass. Upon failure of any of the switchingtransistors, that element of the circuit is bypassed. Also, uponinductor failure, the current bypasses the failed inductor through theparasitic diodes of the transistor used in the boost converter.

FIG. 7 illustrates a power converter, according to aspects of theinvention. FIG. 7 highlights, among others, a monitoring and controlfunctionality of a DC-to-DC converter 705, according to embodiments ofthe present invention. A DC voltage source 701 is also shown in thefigure. Portions of a simplified buck and boost converter circuit areshown for the converter 705. The portions shown include the switchingtransistors 728, 730, 748 and 750 and the common inductor 708. Each ofthe switching transistors is controlled by a power conversion controller706.

The power conversion controller 706 includes the pulse-width modulation(PWM) circuit 733, and a digital control machine 730 including aprotection portion 737. The power conversion controller 706 is coupledto microcontroller 790, which includes an MPPT module 719, and may alsooptionally include a communication module 709, a monitoring and loggingmodule 711, and a protection module 735.

A current sensor 703 may be coupled between the DC power source 701 andthe converter 705, and output of the current sensor 703 may be providedto the digital control machine 730 through an associated analog todigital converter 723. A voltage sensor 704 may be coupled between theDC power source 701 and the converter 705 and output of the voltagesensor 704 may be provided to the digital control machine 730 through anassociated analog to digital converter 724. The current sensor 703 andthe voltage sensor 704 are used to monitor current and voltage outputfrom the DC power source, e.g., the solar panel 701. The measuredcurrent and voltage are provided to the digital control machine 730 andare used to maintain the converter input power at the maximum powerpoint.

The PWM circuit 733 controls the switching transistors of the buck andboost portions of the converter circuit. The PWM circuit may be adigital pulse-width modulation (DPWM) circuit. Outputs of the converter705 taken at the inductor 708 and at the switching transistor 750 areprovided to the digital control machine 730 through analog to digitalconverters 741, 742, so as to control the PWM circuit 733.

A random access memory (RAM) module 715 and a non-volatile random accessmemory (NVRAM) module 713 may be located outside the microcontroller 790but coupled to the microcontroller 790. A temperature sensor 779 and oneor more external sensor interfaces 707 may be coupled to themicrocontroller 790. The temperature sensor 779 may be used to measurethe temperature of the DC power source 701. A physical interface 717 maybe coupled to the microcontroller 790 and used to convert data from themicrocontroller into a standard communication protocol and physicallayer. An internal power supply unit 739 may be included in theconverter 705.

In various aspects of the invention, the current sensor 703 may beimplemented by various techniques used to measure current. In one aspectof the invention, the current measurement module 703 is implementedusing a very low value resistor. The voltage across the resistor will beproportional to the current flowing through the resistor. In anotheraspect of the invention, the current measurement module 703 isimplemented using current probes which use the Hall Effect to measurethe current through a conductor without adding a series resistor. Aftertranslating the current to voltage, the data may be passed through a lowpass filter and then digitized. The analog to digital converterassociated with the current sensor 703 is shown as the A/D converter 723in FIG. 7. Aliasing effect in the resulting digital data may be avoidedby selecting an appropriate resolution and sample rate for the analog todigital converter. If the current sensing technique does not require aseries connection, then the current sensor 703 may be connected to theDC power source 701 in parallel.

In one aspect of the invention, the voltage sensor 704 uses simpleparallel voltage measurement techniques in order to measure the voltageoutput of the solar panel. The analog voltage is passed through a lowpass filter in order to minimize aliasing. The data is then digitizedusing an analog to digital converter. The analog to digital converterassociated with the voltage sensor 704 are shown as the A/D converter inFIG. 7. The A/D converter 724 has sufficient resolution to generate anadequately sampled digital signal from the analog voltage measured atthe DC power source 701 that may be a solar panel.

The current and voltage data collected for tracking the maximum powerpoint at the converter input may be used for monitoring purposes also.An analog to digital converter with sufficient resolution may correctlyevaluate the panel voltage and current. However, to evaluate the stateof the panel, even low sample rates may be sufficient. A low-pass filtermakes it possible for low sample rates to be sufficient for evaluatingthe state of the panel. The current and voltage date may be provided tothe monitoring and logging module 711 for analysis.

The temperature sensor 779 enables the system to use temperature data inthe analysis process. The temperature is indicative of some types offailures and problems. Furthermore, in the case that the power source isa solar panel, the panel temperature is a factor in power outputproduction.

The one or more optional external sensor interfaces 707 enableconnecting various external sensors to the converter 705. Externalsensors are optionally used to enhance analysis of the state of thesolar panel 701, or a string or an array formed by connecting the solarpanels 701. Examples of external sensors include ambient temperaturesensors, solar radiance sensors, and sensors from neighboring panels.External sensors may be integrated into the converter 705 instead ofbeing attached externally.

In one aspect of the invention, the information acquired from thecurrent and voltage sensors 703, 704 and the optional temperature andexternal sensors 705, 707 may be transmitted to a central analysisstation for monitoring, control, and analysis using the communicationsinterface 709. The central analysis station is not shown in the figure.The communication interface 709 connects a microcontroller 790 to acommunication bus.

The communication bus can be implemented in several ways. In one aspectof the invention, the communication bus is implemented using anoff-the-shelf communication bus such as Ethernet or RS422. Other methodssuch as wireless communications or power line communications, whichcould be implemented on the power line connecting the panels, may alsobe used. If bidirectional communication is used, the central analysisstation may request the data collected by the microcontroller 790.Alternatively or in addition, the information acquired from sensors 703,704, 705, 707 is logged locally using the monitoring and logging module711 in local memory such as the RAM 715 or the NVRAM 713.

Analysis of the information from sensors 703, 704, 705, 707 enablesdetection and location of many types of failures associated with powerloss in solar arrays. Smart analysis can also be used to suggestcorrective measures such as cleaning or replacing a specific portion ofthe solar array. Analysis of sensor information can also detect powerlosses caused by environmental conditions or installation mistakes andprevent costly and difficult solar array testing.

Consequently, in one aspect of the invention, the microcontroller 790simultaneously maintains the maximum power point of input power to theconverter 705 from the attached DC power source or solar panel 701 basedon the MPPT algorithm in the MPPT module 719 and manages the process ofgathering the information from sensors 703, 704, 705, 707. The collectedinformation may be stored in the local memory 713, 715 and transmittedto an external central analysis station In one aspect of the invention,the microcontroller 790 uses previously defined parameters stored in theNVRAM 713 in order to operate. The information stored in the NVRAM 713may include information about the converter 705 such as serial number,the type of communication bus used, the status update rate and the ID ofthe central analysis station. This information may be added to theparameters collected by the sensors before transmission.

The converters 705 may be installed during the installation of the solararray or retrofitted to existing installations. In both cases, theconverters 705 may be connected to a panel junction connection box or tocables connecting the panels 701. Each converter 705 may be providedwith the connectors and cabling to enable easy installation andconnection to solar panels 701 and panel cables.

In one aspect of the invention, the physical interface 717 is used toconvert to a standard communication protocol and physical layer so thatduring installation and maintenance, the converter 705 may be connectedto one of various data terminals, such as a computer or PDA. Analysismay then be implemented as software which will be run on a standardcomputer, an embedded platform or a proprietary device.

The installation process of the converters 705 includes connecting eachconverter 705 to a solar panel 701. One or more of the sensors 703, 704,705, 707 may be used to ensure that the solar panel 701 and theconverter 705 are properly coupled together. During installation,parameters such as serial number, physical location and the arrayconnection topology may be stored in the NVRAM 713. These parameters maybe used by analysis software to detect future problems in solar panels701 and arrays.

When the DC power sources 701 are solar panels, one of the problemsfacing installers of photovoltaic solar panel arrays is safety. Thesolar panels 701 are connected in series during the day when there issunlight. Therefore, at the final stages of installation, when severalsolar panels 701 are connected in series, the voltage across a string ofpanels may reach dangerous levels. Voltages as high as 600V are commonin domestic installations. Thus, the installer faces a danger ofelectrocution. The converters 705 that are connected to the panels 701may use built-in functionality to prevent such a danger. For example,the converters 705 may include circuitry or hardware of software safetymodule that limits the output voltage to a safe level until apredetermined minimum load is detected. Only after detecting thispredetermined load, the microcontroller 790 ramps up the output voltagefrom the converter 705.

Another method of providing a safety mechanism is to use communicationsbetween the converters 705 and the associated inverter for the string orarray of panels. This communication, that may be for example a powerline communication, may provide a handshake before any significant orpotentially dangerous power level is made available. Thus, theconverters 705 would wait for an analog or digital release signal fromthe inverter in the associated array before transferring power toinverter.

The above methodology for monitoring, control and analysis of the DCpower sources 701 may be implemented on solar panels or on strings orarrays of solar panels or for other power sources such as batteries andfuel cells.

Use of Battery as DC Power Source/Sink

A typical rechargeable battery may be made with serially connectedsecondary cells and in some cases, several parallel strings of seriallyconnected cells. Serially connected secondary cells are used to build abattery voltage high enough to fit a specific application voltage. Atypical generic charging application applied to a rechargeable battery,may include a bulk power source which provides raw DC power to therechargeable battery and a regulator which regulates current and/orvoltage applied to the rechargeable battery. For less-expensivechargers, the regulator is usually a power transistor or otherlinear-pass element that dissipates power as heat. The regulator mayalso be a buck switching supply that includes a standard freewheelingdiode for average efficiency or a synchronous rectifier for highestefficiency. The typical generic charging application may further includea current-control loop which limits the maximum current delivered to thebattery, and a voltage loop which maintains a constant voltage on thebattery. (Note that Li+ cells typically require a high level ofprecision in the applied charging voltage.) Also the current-voltage(I-V) characteristic may be fully programmable, or may be programmablein current only, with a voltage limit (or vice versa). Cell temperatureof the battery may be measured, and charge termination can be basedeither on the level or the slope of this measurement. Charging time maybe measured, usually as a calculation in an intelligence block such asmicroprocessor with memory for example. The intelligence block providesintelligence for the system and typically implements a state machine.The intelligence block using the state machine knows how and when toterminate a charge. Discharge is done, usually, directly from a cellarray, via current sensing (in order to keep track of actual batterycharge).

A serial connection of battery cells may pose a challenge in managingthe charge and discharge of battery cells. All cells typically must bematched in terms of electrical characteristics and initial chargelevels. Cells also need to be matched thermally otherwise the sameelectrical conditions can have different (and catastrophic) results fordifferent cells Usually several temperature sensors are used to meet theneeded safety requirements but the typical outcome is that the entirebattery charge performance is limited by the weakest cell. Addingseveral parallel strings of cells may be an additional challenge, sinceimpedance of all cells arc low, any small impedance difference mayresult in a large variance in current between strings. The largevariance in current between strings may be difficult to manage withoutsome separate circuit hardware per string.

Reference is now made to FIG. 3a which show system 30 a according to anembodiment of the present invention. System 30 a includes converters 305a-305 d with terminals connected in series to form a string 3003, thesame as shown in configuration 30 (FIG. 3). The other terminals ofconverters 305 a-305 d are connected to re-chargeable cells 6001 a-6001d respectively to form a module 320 a. Each module 320 a includes acontrol loop 323 i that receives a feedback signal, from the connectionbetween a converter 305 and a cell 6001. Loop 323 i typically determinesthe voltage across the connection between converter 305 and cell 6001and/or the current between converter 305 and cell 6001. String 3003 isconnected to a terminal of power controller 3004 a. Several strings 3003may be further connected to the terminal of power controller 3004 a byconnecting strings 3003 in parallel. The other terminal 330 of powercontroller 3004 may be connected to a power supply or a load. The powersupply may be an AC supply such as a grid voltage or a DC supply. Theload may be an AC load or a DC load. System 30 a typically operates intwo modes. One mode is the discharge of cells 6001 to supply the load orthe power supply connected to power controller 3004 a. The other mode isto charge cells 6001 via power controller 3004 a when controller 3004 ais connected to the power supply. During charging of cells 6001,controller 3004 a typically operates as a parallel charger. Controller3004 a acts as a voltage source that supplies any amount of power up tothe total power available by the power source. The voltage source can befixed to almost any voltage and can be optimized depending on the amountof modules 320 a. Power controller 3004 a may be DC to AC inverter or aDC to DC converter the same as a converter 305 for example. According toa feature of the present invention an independent control loop 321 a ofcontroller 3004 a typically holds the voltage of string 3003 at a setvalue.

Reference is now made to FIG. 3b which shows system 30 b according to anembodiment of the present invention. System 30 a includes converters 305a-305 d with terminals connected in series to form a string 3003, thesame as show in configuration 30 a (shown in FIG. 3a ). The otherterminals of converters 305 a-305 d are connected to re-chargeable cells6001 a-6001 d respectively to form a module 320 a. Each module 320 aincludes two control loops 323 i and 323 o. Converter 305 mayindependently choose which loop 323 i or 323 o to use for operation ofconverter 305. Converter 305 may also operate control loops 323 i and323 o simultaneously, such that loop 323 i determines the voltage andcurrent of battery 6001 and hence the power (P) of battery 6001. Whilstat the same time, loop 323 o determines the voltage and current ofconverter 305 and therefore power in string 3003. The voltages andcurrents on either side of converter 305 change respectively in order topreserve maximum power through converter 305. Converter 3004 a typicallymay be another DC-DC converter 305 without loops 323 i and 323 o or maybe a DC-AC inverter. Loop 323 i provides a feedback signal to converter305, from the connection between converter 305 and cell 6001. Loop 323 itypically determines the voltage across the connection between converter305 and cell 6001 and/or the direction of current (i.e. charging ordischarging) between converter 305 and cell 6001. Loop 323 o provides afeedback signal to converter 305 from string 3003. Loop 323 o typicallydetermines the voltage contribution of converter 305 to string 3003and/or the direction of current to converter 305.

String 3003 is connected to converter 3004 a. Several strings 3003 maybe further connected to converter 3004 a by connecting strings 3003 inparallel. The other side 330 of converter 3004 may be connected to apower supply or a load. The power supply may be an AC supply such as agrid voltage or a DC supply. The load may be an AC load or a DC load.System 30 b typically operates in two modes. One mode is the dischargeof cells 6001 to supply the load or the power supply connected toconverter 3004 a. The other mode is to charge cells 6001 via converter3004 a connected to the power supply. During charging of cells 6001,converter 3004 a typically operates as a simplified parallel charger.Converter 3004 a acts as a voltage source that supplies any amount ofpower up to the total power available by the power source. The voltagesource can be fixed to almost any voltage and can be optimized dependingon the number of modules 320 a.

Reference is now made to FIG. 3c which show system 30 c according to anembodiment of the present invention. System 30 c includes converters 305a-305 d with terminals connected in series to form a string 3003, thesame as show in configuration 30 a (shown in FIG. 3a ). The otherterminals of converters 305 a-305 d are connected to re-chargeable cells6001 a-6001 d respectively to form a module 320 a. Each module 320 aincludes battery 6001, converter 305, and control loop 323 i. Serialstring of modules 320 a is connected in parallel with a second serialstring 303 of modules 320 including photovoltaic panel 301, andconverter 305 each with control loop 323 i. The two serial strings areconnected to power converter 3004 a which may be a grid connected DC-ACinverter or DC-DC converter on connection 330 for instance. Powerconverter 3004 a is shown with a control loop 321 a which sets thevoltage or current in the parallel-connected serial strings at apreviously determined value generally dependent on the direction ofcurrent flow, i.e. charging or discharging. System 30 c may be used forgrid-connected or off grid applications. In particular, photovoltaicmodule string 303 may be used to charge batteries of battery string3003. The energy stored in battery string 3003 may be sold to the gridat a later time for instance when the electricity price tariffs arehigher.

During charging a converter 305 acts as an optimized charger for abattery 6001. Charging a battery 6001 is preferably performed bycontrolling the current (I)/voltage 30 (V) characteristics of a chargeprofile for a converter 305, to feed into a battery 6001 the mostfavorable charge power needed at any given time. Converter 305 may alsoact as a current source, perform voltage regulation or trickle chargedepending on need. One side of each converter 305 may draw a differentpower from controller 3004 a, depending on power needed by a battery6001 connected to the other side of converter 305. By sharing the samebattery string 3003 current, the voltage of each converter 305 inbattery string 3003 will typically be different for each converter 305.The total voltage provided by string 3003 will typically be the voltageset by controller 3004 a. If a battery is fully charged, converter 305will enter a bypass mode in which it is not taking power from controller3004 a. Controller 3004 a may also turn ON or OFF any number ofconverters 305 in case controller 3004 a does not have enough power tocharge all batteries 6001. In an optimal way, controller 3004 a can shutOFF some of converters 305 leaving only some of batteries 6001 to becharged. Once batteries 6001 are fully charged converters 305 will shutOFF and other batteries 6001 can be charged. By charging some batteries6001 and not other batteries 6001 means batteries 6001 are alwayscharged in the most efficient way independent of the amount of poweravailable for charging. Controller 3004 a communicates with converters305 via power line communication so additional wires are not needed forcharge control of batteries 6001.

Discharge of batteries 6001 is very similar to harvesting power fromdifferent rated photovoltaic modules (PV) modules 301 and/or PV strings303. Controller 3004 a regulates the string 3003 voltage to a fixedvoltage. Each converter 305 will discharge the power in battery 6001.Controller 3004 a may increase or decrease the total amount of powerdrawn via communication with converters 305 so that the total powersupplied is equal to the load needed. Each converter 305 will supply theenergy available from its battery 6001. By sharing the same string 3003current, the voltage of each converter 305 on the side connected tocontroller 3004 a will be different for each converter 305. The totalvoltage across string 3004 a is typically set by controller 3004 a.

Reference is now made to FIG. 6a which shows a slightly modified DC-DCconverter 305 based on the DC-DC converter 605 shown in FIG. 6,according to an embodiment of the present invention. Converter 305additionally includes high frequency transducer 62 and low frequencytransducer 63 placed in negative power line 614. Transducers 62 and 63typically include analogue to digital converters and means for powerline communication or wireless communication. Transducer 62 (operativelyattached to controller 606) typically senses current in the higherfrequency portion of converter 305 where switches 628, 626, 648, and 646are typically switching at a high frequency. The sensed current oftransducer 62 is typically conveyed by transducer to controller 606 bywireless communication or power line communication. Transducer 63(operatively attached to controller 606) typically includes monitoringof current, temperature of battery 6001. The sensed current, temperatureof battery 6001 are also conveyed to controller 606 using wirelesscommunication or power line communication. Controller 606 typicallyincludes a microprocessor with memory. Converter 305 connects to battery6001 with positive node 616 and negative node 614. The other end ofconverter 305 has lines 614 and 612. Multiple lines 614 and 612 ofmultiple converters 305 are typically joined to together in series, byconnecting a line 614 of one converter with a line 612 of anotherconverter 305 to form a battery string 3003. A typical bypass routebetween power lines 612 and 610 of a converter 305 may be to haveswitches 650 and 644 ON and switches 630 and 628 OFF. Converter 305 is aBuck-Boost topology power converter that has the ability to control itstransferred I-V curve. The topology of the converter 305 is basicallysymmetrical thus enabling converter 305 to convert power in eitherdirection.

The definite articles “a”, “an” is used herein, such as “a powerconverter”, “a control loop” have the meaning of “one or more” that is“one or more power converters” or “one or more control loops”.

Although selected embodiments of the present invention have been shownand described, it is to be understood the present invention is notlimited to the described embodiments. Instead, it is to be appreciatedthat changes may be made to these embodiments without departing from theprinciples and spirit of the invention, the scope of which is defined bythe claims and the equivalents thereof.

The invention claimed is:
 1. A system comprising: a plurality of powerconverters, each power converter coupled to a corresponding DC powersource of a plurality of DC power sources, wherein each power convertercomprises: control circuitry configured to provide a feedback signal tothe power converter from a connection between the power converter andthe corresponding DC power source coupled to the power converter; and apower conversion portion configured to charge or discharge thecorresponding DC power source coupled to the power converter, whereineach power converter of the plurality of power converters is connectedin series to at least one other power converter of the plurality ofpower converters.
 2. The system of claim 1, wherein the controlcircuitry in each power converter of the plurality of power convertersis configured to set a voltage between or current through terminals ofthe power converter.
 3. The system of claim 1, wherein the controlcircuitry in each power converter of the plurality of power convertersis configured to lock an input voltage and current from thecorresponding DC power source coupled to that power converter to anoptimal power point.
 4. The system of claim 1, wherein each powerconverter of the plurality of power converters is configured to functionas one or more of a current source, a voltage regulator, or a tricklecharge source.
 5. The system of claim 1, wherein a first DC power sourceof the plurality of DC power sources comprises a DC battery.
 6. Thesystem of claim 1, wherein a first DC power source of the plurality ofDC power sources comprises a solar panel.
 7. The system of claim 1,wherein a first power converter of the plurality of power converters iscoupled to a first DC power source, the first DC power source comprisinga plurality of solar cells connected in series, or a plurality of solarcells connected in parallel, or a plurality of solar cells connected inboth series and parallel.
 8. The system of claim 1, wherein theplurality of power converters are configured to form a serial string ofpower converters, and wherein the system further comprises: a powercontroller coupled to the serial string of power converters, the powercontroller configured to maintain current through or voltage across theserial string of power converters at a regulated value.
 9. The system ofclaim 8, wherein the power controller comprises one or more of abi-directional DC/AC inverter, a bi-directional DC/DC converter, acharging regulator, or a load controller.
 10. The system of claim 9,further comprising a shunt regulator coupled to the serial string ofpower converters and to the power controller.
 11. A system comprising: afirst plurality of DC power sources; a first plurality of powerconverters, wherein each power converter of the first plurality of powerconverters is coupled to a corresponding DC power source of the firstplurality of DC power sources, wherein each power converter of the firstplurality of power converters comprises control circuitry and a powerconversion portion, and wherein each power converter of the firstplurality of power converters is serially connected to at least oneother power converter of the first plurality of power converters,thereby forming a first serial string; a second plurality of DC powersources; and a second plurality of power converters, wherein each ofsecond plurality of power converters is coupled to a corresponding DCpower source of the second plurality of DC power sources, wherein eachpower converter of the second plurality of power converters comprisescontrol circuitry and a power conversion portion, and wherein each powerconverter of the second plurality of power converters is seriallyconnected to at least one other power converter of the second pluralityof power converters, thereby forming a second serial string, wherein thefirst serial string and the second serial string are connected inparallel to form parallel-connected strings.
 12. The system of claim 11,wherein the control circuitry in each power converter of the firstplurality of power converters and each power converter of the secondplurality of power converters is configured to provide a feedback signalto the power converter from a connection between the power converter andthe corresponding DC power source coupled to the power converter. 13.The system of claim 11, wherein the first plurality of DC power sourcescomprise one or more batteries.
 14. The system of claim 13, wherein thesecond plurality of DC power sources comprise one or more photovoltaicpanels.
 15. The system of claim 11, further comprising: a powercontroller coupled in parallel to the parallel-connected strings, thepower controller configured to maintain current through or voltageacross the parallel-connected strings at a first value.
 16. The systemof claim 15, wherein the power controller is further configured tocontrol a direction of the current and to charge the first plurality ofDC power sources from the second plurality of DC power sources.
 17. Thesystem of claim 16, wherein the power controller is further configuredto: determine that an amount of power received from the second pluralityof DC power sources is insufficient to charge all of the first pluralityof DC power sources; and based on the determination and when chargingthe first plurality of DC power sources, turn off one or more of thefirst plurality of power converters, while leaving on the remainingpower converters in the first plurality of power converters.
 18. Thesystem of claim 16, wherein each power converter of the first pluralityof power converters is configured to: during the charging of the firstplurality of DC power sources by the power controller, determine whetherthe corresponding DC power source coupled to that power converter isfull; and based on a determination that the corresponding DC powersource is full, enter a bypass mode in which power is not taken from thepower controller by that power converter.
 19. The system of claim 16,wherein the first plurality of power converters are configured to drawdifferent amounts of power from the power controller based on powerneeds of the plurality of DC power sources coupled to the firstplurality of power converters.
 20. A method, comprising: coupling aplurality of DC power sources respectively to a plurality of powerconverters, wherein each power converter of the plurality of powerconverters comprises: a control circuit configured to provide a feedbacksignal to the power converter from a connection between the powerconverter and the DC power source coupled to that power converter; and apower conversion portion configured to convert power to discharge orcharge the DC power source coupled to that power converter; coupling theplurality of power converters together in a serial connection to form aserial string of power converters; and coupling the serial string ofpower converters to a power controller configured to maintain currentthrough or voltage across the serial string of power converters at a setvalue.